A class D amplifier enables extremely good power conversion efficiency to be obtained as compared with an analog linear amplifier such as class A and class B amplifiers. Thanks also to its low heat discharge, the class D amplifier is widely used as a speaker drive amplifier. The class D amplifier is operated by causing an amplification active element such as a transistor to perform switching on an input signal such as an audio signal. The class D amplifier performs digital modulation of a PWM (pulse width modulation) signal based on an input audio signal, performs power amplification for the PWM signal, and supplies the power-amplified PWM signal to a speaker.
In PTL 1, a digital amplifier that drives a speaker is disclosed. The digital amplifier disclosed in PTL 1 obtains digital pulse output by performing signal processing and performing digital pulse width modulation on a digital analog signal. Then, this digital amplifier corrects distortion by feeding back the state of the output stage for pulse waveform edge delay, and achieves a reduction in distortion in a part in which a digital pulse that drives the speaker has been converted to an analog signal.
The digital amplifier disclosed in PTL 1 performs demodulation from digital amplifier output passed through a low-pass filter to an analog signal. The digital amplifier employs the above-described method as a method of improving distortion of an analog signal that drives a speaker.